The present invention is applicable to growing any photosynthetic organism, i.e. any life form likely to develop and photosynthesize in a suitable nutritional culture medium, in the presence of solar radiation and carbon-rich gas, such as carbon dioxide, microalgae being the primary representatives of this life form.
The analysis of the compared performance of the photosynthetic organisms leads to favoring the growth of microalgae, which are the oldest players in photosynthesis on our planet. Unlike upright plants, they do not have any complex member with a long construction time to access water and light. They do not need to stiffen their stems using metabolites (cellulose, lignin) that are difficult to decompose. Increased effectiveness of the microalgae cultures follows, translating to surface productivities that can reach 100 tons (dry weight) per hectare and per year versus 10 tons for the best field plants. All of the biomass is usable, whereas large-scale farming harvests generally only concern grains, with the exception in particular of sugarcane and forage plants that leave at least the root system in place.
More particularly, the photosynthetic microorganisms concerned by the invention include aquatic plants such as, for example, microalgae, moss protonemas, small microalgae, and isolated cells of multicellular plants. These aquatic plants have interesting properties in particular in fields such as pharmacy, human and animal nutrition, dermo-cosmetology, energy, and the environment.
As for most photosynthetic microorganisms, access to this resource consists essentially of assisted growth in adapted reactors. Light being their main substrate, the culture medium must have an optical interface receiving a light flow. The difficulty of cultivating photosynthetic microorganisms is related to the fact that they themselves constitute obstacles to the passage of light, which is their main substrate. The growth of the culture will therefore stabilize when the light no longer penetrates the thickness of the culture. This phenomenon is called self-shadowing.
The light path length makes it possible to characterize the different confinement modes, and is defined as:
the length the light travels from its entry into the culture through a transparent optical interface to an opposite opaque wall; or
half the distance separating the two transparent optical interfaces when the confinement receives the light by two opposite transparent optical interfaces.
This light path length varies between a few centimeters and a few decimeters and mainly determines the production of biomass per unit of time and optical surface (surface productivity in g/m2/d) and the concentration of the culture (en g/L) in the final growth phase. The different confinement modes that are implemented to ensure the growth of small aquatic plants can thus be classified as a function of this characteristic length.
The photosynthesis reaction is also accompanied by a consumption of carbon dioxide (CO2) and oxygen production. The excess oxygen inhibits the reaction, while the absence of carbon dioxide interrupts it for lack of substrate to transform. A gas/liquid interface must therefore be developed for mass transfers between these gases and the liquid phase. In order to favor these exchanges and avoid heterogeneities, the culture must be the seat of a mixture intended to renew the organisms at the aforementioned optical interface and also at this gas/liquid interface.
A first known embodiment of a photosynthetic reactor consists of an open container of the basin or tray type where the culture is kept by gravity and has a free surface making the optical interface and the liquid/gas interface by itself. The culture is mixed inside the basin by one or several mechanical stirring devices, for example of the vaned rotor type. The cultures in the basin thus realized can cover significant surfaces and this embodiment is at the origin of the majority of current world microalgae production, which reaches several thousands of tons in dry weight. The photosynthetic organisms produced by this type of reactor are essentially:
so-called extremophilic algae whereof the mediums are hostile to predators and competitors, such as for example algae of the spirulina or Dunaliella type; or
so-called dominant algae that support mechanical stresses or contaminations better than the others, such as, for example, algae of the Chlorella, Scenedesmus, Skeletonema, Odontella or Nannochloropsis types.
A second known embodiment of a photosynthetic reactor also consists of an open container of the tank or tray type, but whereof the dimensions are smaller than those of the basins of the first known embodiment. These containers generally have lateral walls transparent to solar radiation, such that the optical interface is made up both of the free surface of the liquid medium and by the transparent lateral walls.
In this second embodiment, it is traditional to use an injection of air done in the lower portion of the tank, which leads to the formation of air bubbles rising in the liquid up to the free surface. The surface of the bubbles thus formed constitutes the gas/liquid interface. While rising to the surface, the bubbles pull the culture upwards, thereby creating convective movements that can extend to the entire volume. Carbon dioxide (CO2) is sometimes added to the injected air to contribute additional carbon according to a predefined molar fraction of several percent.
The tanks of the second known embodiment, which have a smaller volume than the basins of the first embodiment, are adapted to more controlled cultures, in particular microalgae cultures intended for the nutrition of mollusk larvae or live prey of fish larvae in aquaculture. Frequent cleaning of these tanks as well as pure and massive inoculation make it possible to limit contaminations inside the tank. The microalgae thus grown, of which there are several dozen species, have relatively close temperature and light needs that make it possible to grow them in shared areas.
These two embodiments in the form of an open container offer a light path length of one to several decimeters.
A third known embodiment of a photosynthetic reactor consists of a closed reactor, so-called photobioreactor, comprising a closed loop inside which the liquid culture medium circulates, said closed loop comprising a reaction channel provided with reaction sections made of a material transparent to light radiation (or light), and a closing channel ensuring the connection between the two opposite ends of the reaction channel.
Photobioreactors, described in particular in documents GB 2 118 572 A, ES 2 193 860 A1, GB 2 331 762 A, ES 2 150 389 A1, FR 2 685 344 A1 and FR 2 875 511 A3, offer substantially smaller light path lengths, in the vicinity of one to several centimeters, in relation to the embodiments with an open container, and they make it possible to achieve concentrations of photosynthetic organisms of several grams per liter sheltered from airborne contamination. The reaction channel of the photobioreactors generally consists of transparent plates or tubes, made of glass or plastic, with a thickness or diameter in the vicinity of a centimeter, that are connected end to end by bends to form a winding channel together.
The closing channel comprises a so-called ascending vertical tube, in which the liquid medium rises, and a descending vertical tube in which the liquid medium descends under the effect of gravity.
The gas injection system generally implemented in photobioreactors consists of an airlift, also called gas-lift, i.e. through a gas injection at the base of an ascending vertical tube of the closing channel, said gas injection serving both to circulate or move the liquid reaction medium and perform the gas-liquid exchanges. The gas-lift includes, in the upper portion, a widened gravity tank or volume in which the lower circulation speeds allow the gas-liquid separation, and the descending vertical tube of the closing channel emerges in the bottom of the gravity tank to supply the reaction channel with liquid.
The aforementioned photobioreactors apply the principle that the reaction only takes place in the liquid phase, in other words these photobioreactors seek to minimize the volume of gas injected into the reactor so as not to decrease the volume of the liquid culture medium by as much, out of a concern for not decreasing production. Thus, in these photobioreactors, oxygen is extracted using a vertical ascending tube defined above, said vertical ascending tube forming an air bubble column emerging in the gravity tank receiving the liquid culture medium, and including a gas injection in the lower portion, opportunely of CO2-enriched air. As described above, the circulation and gas transfer functions are combined within this single device, called gas-lift, which creates an ascending vertical circulation by movement quantity exchange between the liquid mass and the gas bubbles resulting from the injection. The supersaturated photosynthetic oxygen in the liquid moves to the gas phase by air sweeping, while the CO2 goes to solution. These degassing and carbonation phases are essential and take place simultaneously and indissociably at this single device in which the culture must pass at a high frequency to prevent a deleterious increase in the dissolved oxygen content.
Gas-lifts have the drawback of generating gas bubbles that rise in the vertical ascendant tube of the closing channel of the photobioreactors. The applicant has in fact observed the deleterious role of these bubbles for the microorganism culture in the photobioreactors:
on one hand, the bubbles mechanically stress the microalgae and can harm fragile microorganisms; and
on the other hand, the bubbles capture, through tensioactive effect, the molecules that have tensioactive properties, and in particular organic molecules, cellular debris, and the excretion products of living cells. These substances, normally dispersed in the medium in the absence of bubbles, are thus assembled in the form of aggregates on the free surface of the gravity tank when the bubbles burst. The bacteria and fungi that would not be able to develop due to the strong dilution of these organic molecules then find concentrated substrates favorable to their development.